The disclosure incorporates the bushingless piston and connecting rod assembly and method of manufacture disclosed in provisional application 60/168,352, filed Dec. 1, 1999, whose priority is claimed for this application.
1. Technical Field
This invention relates generally to pistons for heavy duty diesel engine applications.
2. Related Prior Art
The small end of steel connecting rods for heavy duty diesel applications typically has a Cuxe2x80x94Pbxe2x80x94Sn bushing pressed therein in order to provide tribological compatibility between the steel connecting rod and a steel wrist pin installed in the small end for coupling the connecting rod to an associated piston. Diesel engines are known to generate a corrosive, abrasive operating environment around such bushings. As emission regulations become more stringent to reduce Nox generation in the cylinders, it is expected that the levels of corrosive environmental constituents such as sulfur, nitrous and weak organic acids condensation will increase within the cylinder on the order of about 20 to 30% and attack such Cu-based bushings. In addition, ashes resulting from the wear of liners and rings and the lubricant combustion will likely be ingested with the air stream, such that the environment around the bushing will become much more corrosive to standard Cuxe2x80x94Pbxe2x80x94Sn bushings than the environments they presently operate under. Moreover, the ever increasing demand for greater performance of engines is expected to increase combustion pressures to 180 bar and beyond, exceeding the load capacity of the traditional Cuxe2x80x94Pbxe2x80x94Sn bushings. Similar concerns arise with respect to the Cu-based bushings traditionally used between the wrist pin and the body of the piston.
It is an object of the present invention to overcome such problems by eliminating the need for such bushings through the use of engineered coatings.
A piston assembly for an internal combustion engine includes a piston body formed with a cross bore, a steel connecting rod having a cross bore with a steel running surface aligned with the cross bore of the piston body, and a steel wrist pin disposed in the aligned bores and coupling the piston body and the connecting rod and having a steel running surface. According to the invention, a coating of manganese phosphate is applied to at least one of said steel running surfaces of the wrist pin and the connecting.
The coating absorbs and traps lubricating oil and develops a stiff lubricant squeeze film between the mating running surfaces of the connecting rod and wrist pin to provide the necessary tribological properties, eliminating the need for a conventional Cu-based bushing.
The invention has the further advantage of overcoming the concerns of the more aggressive operating environment, since the parent steel materials and the thin coating are inherently resistant to such constituents and retain their properties at elevated temperatures.
The invention has the further advantage of minimizing or eliminating the concern over bushing wear from increased levels of abrasive contaminants. The steel pins and connecting rods along with the coating are resistant to wear from such contaminants.
By eliminating the bushing, a cost savings is also recognized in both the material and labor of installation along with a reduction in the weight of the piston assembly.
According to a further aspect of the invention, the same coating principle is used between the wrist pin and cross bore surface in the piston body to eliminate the need for the normal bushings. The same advantages apply.